The manufacture of shaped polycrystalline silicon carbide articles is, however, not without difficulty. It is known that, without the concomitant use of sintering aids, drastic conditions are required to produce dense shaped articles from pure silicon carbide powder. Manufacture of polycrystalline articles from pure silicon carbide which approach the true density of the material requires a temperature in the range of 3500.degree. C. to 4000.degree. C. and simultaneously, a pressure of 689 MPa (100,000 psi) ( see U.S. Pat. No. 3,158,442) or the conditions for diamond synthesis (see U.S. Pat. No. 3,667,911).
Because of the poor sinterability of silicon carbide powder, various additives have been used as sintering aids to provide moderately dense to dense, shaped articles. The density of the shaped articles is dependent on the process used in each case. High density alone, however, is not the sole criterion for predicting excellent properties in the shaped articles. Density is not a good predictor for high temperature flexural strength which should remain satisfactory at high temperatures that is, up to at least 1500.degree. C.
One of the early processes for the manufacture of dense shaped silicon carbide articles is based on reaction sintering, in which silicon carbide powder is processed with carbon and/or an organic resin binder to form preshaped articles which are subsequently heated in the presence of an atmosphere containing silicon. In the process, the carbon reacts with the silicon to form additional SiC thus bonding the SiC grains already present. Pores are simultaneously filled by infiltration with excess silicon. Although the sintered articles obtained in that manner are substantially pore free and have a very high density, they contain free silicon. Narrow limits are therefore set on their use in high temperature machines since at temperatures exceeding 1400.degree. C., they tend to exude silicon (m.p. 1440.degree. C.).
Dense shaped silicon carbide articles can also be manufactured by the conventional hot-pressing or pressure-sintering process using graphite molds and additives containing aluminium or boron as sintering aids.
The best results which could hitherto be achieved by means of the hot-pressing or pressure-sintering process were shaped articles consisting of polycrystalline .alpha.- or .beta.-silicon carbide having a density of at least 99% of the theoretical density of silicon cabide (abbreviated hereinafter as %TD). The shaped articles were manufactured by hot-pressing, with simultaneous shaping from .alpha.- or .beta.-SiC powder together with small amounts of an additive containing aluminium such as aluminium powder, aluminium nitride or aluminium phosphide, at temperatures of up to 2200.degree. or 2300.degree. C. using a die pressure of at least 100 bar (10 MPa).
The shaped articles manufactured in that manner have a substantially single-phase microstructure since the additional components are present substantially in the form of a solid solution in the SiC lattice. They have good high-temperature strength, with flexural strengths of from at least 600N/mm.sup.2 at 1450.degree. C. which do not, however, reach values of 700N/mm.sup.2 (see EPO No. 021 239 A 1 and EPO No. 022 522 A 1).
Although the shaped articles consisting of polycrystalline silicon carbide obtained in that manner and having low porosity have good high-temperature properties, the formation, during manufacture, of vitreous alumino-silicate phases at the grain boundaries of the powder being densified cannot be completely eliminated due to the essential concomitant use of an aluminium-containing additive as a sintering aid. The vitreous aluminosilicate phases at the grain boundaries can, even in very low concentrations, adversely affect the physical properties of the articles.
The possibilities of forming articles by hot die pressing are limited since only relatively simple shaped articles can be manufactured. A conventional hot press using die pressure can, in each case, densify simultaneously only one or at most, a few shaped articles.
Attempts have been made to replace the expensive hot-pressing process for forming silicon carbide articles by the pressureless-sintering process.
The best results achieved with the pressureless sintering provided only shaped articles consisting of polycrystalline .alpha.-SiC having less than 99%TD. The shaped articles were formed by cold pressing .alpha.-SiC, in the form of submicron powder, together with an aluminium-containing additive and a carbon-containing additive and then sintering the shaped articles without pressure at a temperature of from 2000.degree. C. to 2300.degree. C.
In the shaped articles manufactured by pressureless sintering, the additional carbon is present at least partially in free elemental form, and appears as a second phase alongside the SiC. The flexural strength of the shaped articles which have a total porosity of, at most 2% by volume, are about 600N/mm2 (see EP O No. 004 031 B1).
The problem therefore is to make available substantially pore-free shaped articles of polycrystalline silicon carbide which have been manufactured, without the concomitant use of sintering aids, from pure SiC powder, without the use of drastic conditions with regard to pressure and temperature, or the conditions for diamond synthesis.